1. Technical Field
This invention relates to the art of synthesizing diamond and hard carbon films and, more particularly, to the art of adhering such films to the working surface of cutting or forming tools.
2. Discussion of the Prior Art
The synthesis of diamond under high pressure and high temperature conditions (HPHT) is a well-known success, originating with its announcement in 1955; several tons of industrial diamond abrasive grain are made this way each year in various factories around the world (see R. H. Wentorf, R. C. DeVries, and F. P. Bundy, "Sintered Superhard Materials", Science, Vol. 208, p. 873 (1980); and J. C. Walmsley, "The Microstructure of Ultrahard Material Compacts Studied by Transmission Electron Microscopy", Proceedings of International Conference on Science of Hard Materials, Nassau, Bahamas, November 1987). Unfortunately, current HPHT synthesis schemes do not permit direct coating of tool materials without detrimentally affecting such tool materials; the cost, inability to cover large areas, and complexity of making controlled diamond by this process is high.
Only in the last few years has it been possible to produce diamond crystals by low pressure chemical vapor deposition (CVD). The possibility of synthesis of diamond under metastable conditions, i.e., in the graphite stable region, is based on the small free energy difference between diamond and graphite under ambient conditions. By using atomic hydrogen during such synthesis, diamond will be favored to deposit from a hydrocarbon vapor. The presence of atomic hydrogen appears to be the key ingredient because it removes graphite or prevents its formation while promoting the crystalization of diamond in the metastable state (see R. Messier, K. E. Spear, A. R. Badzian, and R. Roy, "The Quest For Diamond Coatings", Journal of Metals, Vol. 39 [No. 9], page 8, (1987); and R. C. DeVries, "Synthesis Of Diamond Under Metastable Conditions", Annual Reviews of Materials Science, Vol. 17, page 161, (1987)). The conversion Process, assisted by the use of a variety of thermal techniques including heated filaments and microwave plasma, can be considered for use on some tool materials.
In many of the early investigations, there was little concern for micromorphology and adhesion to a substrate. In later investigations, it was observed that crystals appeared more readily at scratches on the substrate; such substrates were formed of materials familiar to the semiconductor art, such as silicon, copper, tungsten, and molybdenum (see Y. Mitsuda, Y. Kojima, T. Yoshida and K. Akashi, "The Growth of Diamond In Microwave Plasma Under Low Pressure", Journal of Materials Science, Vol. 22, page 1557 (1987); B. V. Spitsyn, L. L. Bouilov and B. V. Derjaguin, "Vapor Growth Of Diamond On Diamond And Other Surfaces", Journal of Crystal Growth, Vol. 52, page 219 (1981); S. Matsumoto, Y. Sato, M. Tsutsumi and S. Setaka, "Growth Of Diamond Particles From Methane-Hydrogen Gas", Journal of Materials Science, Vol. 17, page 3106 (1982); and A. R. Badzian, T. Badzian, R. Roy, R. Messier and K. E. Spear, "Crystallization Of Diamond Crystals And Films By Microwave Assisted CVD (Part II)", Materials Research Bulletin, Vol. 23 , page 531 (1988)).
When research investigators turned their attention to the adhesion of diamond coatings to a substrate, such as for cutting tools, they found the coatings suffered (see Y. Yagi, K. Shibuki and T. Takatsu, "Adhesion Strength Of Diamond Films On Cemented Carbide Substrate", presented at the 15th International Conference on Metallurgical Coatings, April 11-15, San Diego, California (1988)).
In various efforts, the wettability of diamond by metals has been investigated. The affinity of titanium, nickel, cobalt, manganese, chromium, molybdenum, and iron for diamond is made evident by such works as: Yu. V. Naidich and G. A. Kolesnichenko, "Study of the Wetting of Diamond and Graphite by Liquid Metals II. Angles of Contact and Adhesion Between Tin-Titanium and Copper-Tin-Titanium Alloys and the Graphite Surface", Poroshkovava Metallurgiya 1 (13) p. 49 (1963); Yu. V. Naidich and G. A. Kolesnichenko, "Investigation of the Wetting of Diamond and Graphite by Molten Metals and Alloys III. The Wetting of Diamond Crystals", Poroshkovava Metallurgiya 3 (21) p. 23 (1964); Yu. V. Naidich and G. A. Kolesnichenko, "Investigation of the Wetting of Diamond and Graphite by Molten Metals and Alloys IV. Influence of Temperature on the Adhesion of Metals Inert to Carbon", Poroshkovava Metallurgiya 2 (38) p. 97 (1966); and Yu. V. Naidich and G. A. Kolesnichenko, "Investigation of the Wetting of Diamond and Graphite by Molten Metals and Alloys V. Carbide Formation Kinetics at the Graphite/Metallic Melt Interface", Poroshkovaya Metallurgiya 2, p. 76 Feb. 1968. To effect wetting, the above investigations heated the materials to above 1100.degree. C. for time periods of at least 10 minutes duration. Such heating would promote graphitization or dissolution of thin diamond coatings and thus makes the results of such investigations not usable for promoting improved diamond coated tools. If such wetting metals were deposited by chemical vaporization techniques, the temperature of processing (above 600.degree. C.) for a necessary period of time (i.e., 30 minutes) would cause graphitization and/or dissolution of deposited diamond or diamond-like particles. Lower temperature depositions of refractory metals is possible using plasma-activated CVD processes (PACVD), but such technology falls far short of knowing how to obtain adherence to cemented nonmetal substrates and how to convert to a platform that stimulates diamond growth (see J. A. Sheward and W. J. Young, "The Deposition of Molybdenum and Tungsten Coatings on Gun Steel Substrates by a Plasma Assisted CVD Technique", Vacuum, Vol. 36, p. 37 (1986)).
Such accumulated knowledge is not able to provide a tougher, robust, and lower cost diamond coated tool without degrading the effectiveness or cost competitiveness of the tool for finish machining. Such tougher, robust, coated tools must be more resistant to cracking, loss of adhesion, and ultimate spallation; the composite tool must have higher hot strength, better thermal conductivity, and improved accommodation of the widely differing physical Properties of the substrate and diamond or diamond-like particles applied thereto.
An obvious advantage of CVD is the lower cost to fabricate a diamond or diamond-like coating on a tool substrate. The requisite temperature for such CVD, coupled with the necessity that the substrate be capable of forming a carbide to facilitate nucleation of diamond, restricts the selection of substrate materials essentially to transition metals and their carbides, and forms of silicon. In the case of transition metal carbides, the grains are usually cemented together by use of metals or alloys (i.e., Co, Ni, Fe) which, unfortunately, are catalytically active to diamond, causing graphitization and an attendant reduction of properties. Such catalytically-active metals poison the diamond-creating potential of CVD and must be overcome.
The problems facing developers of industrially-robust diamond coated tools remain. One of these is the nature of the CVD process which requires that the substrate be subjected to a temperature of about 1000.degree. C., which basically eliminates the use of many types of tool substrates and restricts the selection to high temperature resistant substrates that generally are not strong in tension. A second problem involves adherence and mechanical strength of the coating in contact with the supporting substrate. There are very few substrate materials, and almost none commercially, which can tolerate both the high temperatures of the current deposition process as well as provide sufficient mechanical support to sustain the internal stresses which are developed in a continuous diamond film due to the exceptionally low coefficient of thermal expansion of diamond and its extremely high modulus of elasticity.